A23E-01 INVITED 13:40h
Do We Understand the Environmental Impacts of an H2 Economy?
The promise of a clean, hydrogen (H2)-fueled transportation sector has been waved in front of the nation by the current administration, the governor of California, and the technologists. A 2004 NAS report on "The Hydrogen Economy" was prepared by the economists and engineers, remarkably lacking any atmospheric scientists or biogeochemists who understand the natural cycle of H2. It is surprising that all of these groups examining a hydrogen economy are secure in the belief that H2 is a pure fuel, safe and harmless to the environment. A thorough well-to-wheel analysis as envisaged by the NAS report is needed to evaluate the environmental costs/benefits of a hydrogen economy (e.g., the energy source for producing H2, the distribution system), but it needs also to include our knowledge of the biogeochemical cycling of H2 in the atmosphere (0.5 ppm or about 175 Tg) and how it might be perturbed. This brief review of the science examines what controls the atmospheric abundance of H2 and how the chemistry of H2 impacts the climate and global air quality. Recent work on hydrogen isotopes, global modeling of sources and sinks, and the stratospheric and climate connections, enables us to more clearly define the budget and impacts of H2, and to place reasonable bounds on the environmental impacts - provided we understand the possible future emissions of H2 and of related direct and indirect greenhouse gases.
A23E-02 13:55h
Assessing Leaks in a Global Hydrogen Infrastructure: Can it Perturb the Natural Hydrogen Cycle?
The potential of hydrogen leaks from implementation of a global hydrogen economy and the impacts related to depletion of stratospheric ozone have been recently discussed in the scientific literature. However, the leak rates by technologies to be used in the hydrogen economy remain ill constrained. Very little data are available regarding hydrogen leak rates during the transfer operations, or at interfaces, and what data are available may not be particularly useful for consumer applications. For example, observed boil-off and venting rates (upwards of 50 percent in some cases) for the liquid hydrogen filling operations for NASA's Space Shuttle launches would not be acceptable for consumer operations, both from a safety and a cost perspective. On the other hand, we know that natural gas production and distribution systems have current leak rates of 1 to 5 percent, which could be reduced somewhat with technology and process improvements. The properties of hydrogen, particularly its high molecular diffusivity and small size, make achieving "zero emissions" of hydrogen improbable. Furthermore, lowering leak rates below a certain threshold will not be cost effective. Understanding the potential impacts of hydrogen on atmospheric chemistry (stratospheric ozone, global oxidative capacity, and air quality) will require an understanding of the possible leak points during production, transportation, storage, delivery, and use of hydrogen. This paper will examine several hydrogen infrastructure scenarios to identify and quantify, to the extent possible, the likely hydrogen emissions in a hydrogen economy. This will be used to make plausible assessments of the effect of a global hydrogen economy on the natural hydrogen cycle and atmospheric chemistry.
A23E-03 14:10h
Effects on Air Pollution and Regional Climate of Producing and Using Hydrogen in Fuel Cells in all U.S. OnroadVehicles
The purpose of this study was to examine the potential effects on U.S. air pollution and regional climate of switching the current U.S. fleet of onroad motor vehicles to hydrogen fuel-cell vehicles, where hydrogen was produced by (1) steam-reforming of methane, (2) wind energy, or (3) coal gasification. An additional scenario in which the U.S. fleet was switched to gasoline-electric hybrid vehicles was also examined. The model used was GATOR-GCMOM, a global-through-urban-scale nested and parallelized gas, aerosol, transport, radiation, general-circulation, mesoscale, and ocean model. U.S. emission data for the baseline case were obtained from the U.S. National Emission Inventory, which considers 370,000 stack and fugitive sources, 250,000 area sources, and 1700 categories of onroad and nonroad vehicular sources (including motorcycles, passenger vehicles, trucks, recreational vehicles, construction vehicles, farm vehicles, industrial vehicles, etc.). Emission inventories for each of the three hydrogen scenarios were prepared following a process chain analysis that accounted for energy inputs and pollution outputs during all stages of hydrogen and fossil-fuel production, distribution, storage, and end-use. Emitted pollutants accounted for included CO, CO2, H2, H2O, CH4, speciated ROGs, NOx, NH3, SOx, and speciated particulate matter. Results from the first scenario suggest that switching vehicles in the U.S. to hydrogen produced by steam-reforming of methane may reduce emission of NOx, reactive hydrocarbons, CO, CO2, BC, NO3-, and NH4+, but increase CH4, H2, and SO2 (slightly).The switch may also decrease O3 over most of the U.S. but short-term near-surfaces increases may occur over low-vegetated cities (e.g., in Los Angeles and along the Boston-Washington corridor) due to loss of NOx that otherwise titrates O3. The switch is also estimated to decrease PAN, HCHO, and several other pollutants formed in the atmosphere. Isoprene may increase since fewer oxidants (OH, O3) will be available to destroy it. Results for the scenarios involving hydrogen from wind and coal gasification, and from the hybrid scenario will also be discussed, as will regional climate effects (including effects of H2O). Findings to date suggest that, even under a worst-case scenario of 10% hydrogen leakage, the conversion of the current fleet to hydrogen-fuel cell vehicles, where hydrogen is generated by steam-reforming of methane, may result in a measurable improvement in U.S. air quality.
A23E-04 14:25h
Molecular hydrogen in a global chemical transport model: Constraints from surface and oceanic cruise observations of H$_{2}$
We present a new simulation of molecular hydrogen using the GEOS-CHEM global model of tropospheric chemistry in order to improve our understanding of the global budget of molecular hydrogen. Primary sources of H$_{2}$ (fossil fuel, biofuel, and biomass burning) used in the model are based on the GEOS-CHEM CO emission inventory scaled with the appropriate emission factors. Secondary sources from photochemical production arise from the photolysis of formaldehyde (resulting from oxidation of methane and volatile organic compounds, VOCs) and account for an estimated 45$%$ of the H$_{2}$ source. There is considerable uncertainty in the source of H$_{2}$ from formaldehyde, because of poor understanding of VOC sources and their yield to form formaldehyde. The main tropospheric sink, accounting for $\sim$80$%$ of the total, is uptake by enzymes in soils with the remainder through oxidation by hydroxyl (OH). There is also major uncertainty in the H$_{2}$ soil sink. In this presentation we will use surface observations of the seasonal cycle, as well as latitudinal and longitudinal gradients of H$_{2}$ from the global CMDL network and oceanic cruises to constrain the budget of H$_{2}$ in the atmosphere. We may also present preliminary simulations of the deuterium component of H$_{2}$ to further constrain the H$_{2}$ budget through differences in the isotopic signatures of sources and sinks.
A23E-05 14:40h
Long-Term Observation of Atmospheric Molecular Hydrogen Using Aircrafts
Temporal and spatial variations of atmospheric molecular hydrogen (H$_{2}$) are observed using aircrafts over Novosibirsk ($55\deg$N, $83\deg$E), West Siberia, Yakutsk ($62\deg$N, $130\deg$E), East Siberia and Sagami-bay ($35\deg$N, $139\deg$E), Japan since 1997. H$_{2}$ mixing ratios in air samples collected in Pylex flasks are measured using a gas chromatograph (HP5890, Agilent Technologies) equipped with Reduction Gas Detector (RGD-2, Trace Analytical). Primary standard gases with their mixing ratios of 400, 500, 600 and 700 ppb were prepared by gravimetric method. Four working standard gases, ranging from 420 to 630 ppb, are calibrated by primary standard approximately once per year. Atmospheric H$_{2}$ mixing ratios observed in this study show clear seasonal variation with their maximum in spring or summer and minimum in late autumn or early winter. The phase of the seasonal change in lower troposphere is 1-3 months earlier than that in upper troposphere. Peak-to-peak amplitude at 1km over Novosibirsk is 40 ppb, while that at 7km is 23 ppb. In Siberia, the largest gradient in H$_{2}$ mixing ratio, with lower values in lower altitude, is observed in early winter when seasonal variation at lower troposphere indicates minimum. On the other hand, atmospheric H$_{2}$ is almost constant along the altitude in spring when lower tropospheric H$_{2}$ shows maximum. Vertical profile of annual mean H$_{2}$ mixing ratio over Siberia shows negative gradient in each observed year, indicating substantial amount of H$_{2}$ is removed by soil deposition in the continental interior. While over Japan, positive gradient is observed in the annual mean H$_{2}$ at the altitudes lower than 2km, suggesting H$_{2}$ emission by fossil fuel combustion enhance atmospheric mixing ratio near the surface.
A23E-06 14:55h
The Stable Isotope Composition of Stratospheric and Mesospheric H$_{2}$
Stratospheric H$_{2}$ is an intermediate in the chain of oxidation reactions of CH$_{4}$, which ends up with water vapor. Due to the similar reaction rates as CH$_{4}$, H$_{2}$ is in a photochemical steady state between production and destruction in view of mixing ratio. Its D/H ratio, however, is not at a steady state, but is enriched in deuterium associated with both the production and destruction reactions. In the upper stratosphere and mesosphere, water vapor is assumed to be a major source of H$_{2}$ by the photolytic reactions. We discuss the deuterium flow among the long-lived hydrogen compounds, CH$_{4}$, H$_{2}$, and H$_{2}$O in the stratosphere and mesosphere based on the recent observations of the mixing ratios and isotopic ratios of H$_{2}$ and CH$_{4}$ in the stratosphere. Air samples were collected by balloon flights in Aires sur l'Adour (France) in October, 2002, and in Kiruna (Sweden) in March and June, 2003. The D/H ratios of H$_{2}$ and CH$_{4}$ increase with altitude gradually, while the H$_{2}$ mixing ratios remain constant. Exception was observed in a winter flight in March. The H$_{2}$ mixing ratios were decreasing at ~18km but the isotopic ratios kept increasing. Above this layer H$_{2}$ rose to 840 ppb concomitant with a drop in the D/H ratio. We focus on this flight for discussion.
A23E-07 INVITED 15:10h
Legumes, N$_{2}$ fixation and the H$_{2}$ cycle
Legume plants such as soybean or pea can form symbiotic, N2 fixing associations with bacteria that exist in root nodules. For every N$_{2}$ fixed, 1 to 3 H$_{2}$ are produced as a by-product of the nitrogenase reaction. Therefore, a typical N$_{2}$ fixing legume crop produces about 200,000 L H$_{2}$ gas (at STP) per hectare per crop season. This paper will summarize our current understanding of the processes leading to H$_{2}$ production in legumes, the magnitude of H$_{2}$ production associated with global cropping systems, and the implications for its production and oxidation on both the legumes and the soils in which they grow. Specific points may include: $\sim$ In symbioses lacking uptake hydrogenase (HUP) activity (thought to be the majority of crop legumes), the H$_{2}$ diffuses into the soil where it is oxidized by soil microbes that grow up around the legume nodules. The kinetic properties of these microbes are very different (higher Km and Vmax) from that of microbes in soils exposed to normal air (ca. 0.5 ppm H$_{2}$); $\sim$ Laboratory studies indicate that 60% of the reducing power from H$_{2}$ is coupled to O$_{2}$ uptake, whereas 40% is coupled to autotrophic CO$_{2}$ fixation. The latter process should increase soil carbon stocks by about 25 kg C/ha/yr; $\sim$ At the site of the nitrogenase enzyme, H$_{2}$ production is autocatalytic such that the higher the H$_{2}$ concentration, the more H$_{2}$ is produced and the less N$_{2}$ fixed. The variable O$_{2}$ diffusion barrier in legumes can act to restrict H$_{2}$ diffusion from the nodule, thereby increasing the relative magnitude of H$_{2}$ production versus N$_{2}$ fixation; $\sim$ Studies to understand why legume symbioses make such an energy investment in H$_{2}$ production have led to the discovery that H$_{2}$ treated soils have improved fertility, supporting the growth and yield of legume and non-legume crops. This observation may account for the benefits of legumes when used in rotation with cereal crops, a phenomenon that has been used by farmers for over 2000 years, but which has remained unexplained. An attempt will be made to position these results and insights in the context of the impact that a future H$_{2}$ economy will have on the H$_{2}$ cycle.
A23E-08 INVITED 15:25h
Continuous measurements of H2 and CO deposition onto soil: a laboratory soil chamber experiment
Hydrogen uptake in soil is the largest single component of the global budget of atmospheric H2, and is the most important parameter for predicting changes in atmospheric concentration with future changing sources (anthropogenic and otherwise). The rate of hydrogen uptake rate by soil is highly uncertain [1]. As a component of the global budget, it is simply estimated as the difference among estimates for other recognized sources and sinks, assuming the atmosphere is presently in steady state. Previous field chamber experiments [2] show that H2 deposition velocity varies complexly with soil moisture level, and possibly with soil organic content and temperature. We present here results of controlled soil chamber experiments on 3 different soil blocks (each ~20 x ~20 x ~21 cm) with a controlled range of moisture contents. All three soils are arid to semi arid, fine grained, and have organic contents of 10-15%. A positive air pressure (slightly higher than atmospheric pressure) and constant temperature and relative humidity was maintained inside the 10.7 liter, leak-tight plexiglass chamber, and a stream of synthetic air with known H2 concentration was continuously bled into the chamber through a needle valve and mass flow meter. H2, CO and CO2 concentrations were continuously analyzed in the stream of gas exiting the chamber, using a TA 3000 automated Hg-HgO reduced gas analyzer and a LI-820 CO2 gas analyzer. Our experimental protocol involved waiting until concentrations of analyte gases in the exiting gas stream reached a steady state, and documenting how that steady state varied with various soil properties and the rate at which gases were delivered to the chamber. The rate constants for H2 and CO consumption in the chamber were measured at several soil moisture contents. The calculated deposition velocities of H2 and CO into the soil are positively correlated with steady-state concentrations, with slopes and curvatures that vary with soil type and moisture level. Increased moisture content is associated with increased deposition velocities at a given steady-state concentration: for every 5 % increase in soil moisture content, the ratio of deposition velocity to steady state concentration increases by 5.13±1.3 x 10-7cm2/s/ppb. Based on these observations, we conclude that uptake rate of H2 and CO in soil increases with increase in soil moisture content over the range characteristic of unsaturated soils, in contrast to the previous observation that increasing soil moisture level from 30% to 60% caused a large drop in hydrogen uptake rate (Yonemura et al., 1999)-a situation encountered during flood or heavy down pour. Our results indicate that the H2 consuming activity of soil is rapidly activated upon wetting and reaches a maximum at about 15% moisture level. This deduction supports the results obtained by several workers (Conrad and Seiler, 1980 ; Moxley and Smith, 1997) who showed that there is an optimum moisture level for microbiological hydrogen and CO uptake in soil. [1] Rahn T., Eiler, J.M., Kitchen, N., Fessenden, J.E. Geo. Res. Letters, (2002): 29(18): art no 1888. [2] Godde, M., Meuser, K., Conrad R. Hydrogen consumption and carbon monoxide production in soils with different properties, Bio Feril Soils (2000) 32:129-134.